This invention relates to a method of managing files in an interchangeable storage medium. More particularly, the invention relates to a file management method for an interchangeable storage medium having a RAM area and a ROM area, in which these areas serve as mutually independent RAM and ROM sections, respectively, and each section has a file management area for storing file management data and a file area for storing files.
In an optical disk, laser light is narrowed down to a very small beam spot having a diameter of about 1 .mu.m to perform recording and playback of information signals. This is advantageous in that recording density is high and memory cost low on a per-bit basis. Moreover, high-speed access is possible and recording/playback can be carried out in a contactless manner. Such optical disks have been put into practical use as high-density large-capacity memories.
Optical disks can be classified broadly into optical disks (ROM disks) on which information is stored in advance and only reproduction is possible, optical disks (RAM disks) that allow information to be both recorded and reproduced, and partial-ROM disks in which a single optical disk has both of the above-mentioned features.
As shown in FIG. 12A, a ROM disk is such that information is recorded as pits 2 in a transparent plastic layer 1, a metal film (e.g., aluminum) 3 is formed on the pit surface as by vapor deposition, and a protective layer 4 is provided on the metal film 3. In a ROM disk of this kind, the signal layer (the pits and metal film) is irradiated with a laser beam LB via an objective lens OL, as illustrated in FIG. 12B. When this done, almost all of the light returns intact from locations devoid of pits, whereas the light is refracted by pits at locations where the pits are present. Only some of the returned light actually returns to the objective lens OL since part of the light falls outside the visual field of the objective lens. Accordingly, the information can be read by using a photodiode to detect the returning light. Thus, with a ROM disk, information is recorded in the form of pits. This is advantageous in that the information is less likely to be damaged in comparison with magnetic recording, and a large quantity of information can readily be produced on a large number of disks by stamping. Such an optical disk is effective as a storage medium for electronic publishing. A shortcoming, however, is that it is not possible for the user to write information such as text on the ROM disk himself.
A RAM disk (a photomagnetic disk) is obtained by coating a disk surface with a magnetic film such as a thin film of TbFeCo. Such a disk utilizes a property according to which the retentiveness necessary for magnetic reversal of the magnetic film diminishes in conformity with a rise in temperature (retentiveness is zero at the Curie point). More specifically, recording and erasure are performed by irradiating the disk with a laser beam to raise the temperature of the disk medium to the vicinity of 200.degree. C., thereby weakening retentiveness, applying a weak magnetic field under this condition and controlling the direction of magnetization. Accordingly, as illustrated in FIG. 13A, an upwardly directed magnetic field is applied by a writing coil 6 under a condition in which the direction of magnetization of a magnetic film 5 is pointed downward. When a portion at which the direction of magnetization is desired to be changed is irradiated with a laser beam LB via an objective lens OL, as shown in FIG. 13B, the direction of magnetization of this portion reverses, i.e., is pointed upward. This makes it possible to record information. When information is read, the magnetic film 5 is irradiated with a laser beam LB having a plane of polarization along the y axis, as illustrated in FIGS. 13C1, 13C2, 13C3, 13C4 and 13C5. When this is done, reflected light LBO, in which the plane of polarization has been rotated by .theta..sub.k in the clockwise direction owing to the magnetic Kerr effect, is obtained in the portion where magnetization is downwardly directed. In the portion where magnetization is upwardly directed, reflected light LB1, in which the plane of polarization has been rotated by .theta..sub.k in the counter-clockwise direction owing to the magnetic Kerr effect, is obtained. Accordingly, the direction of magnetization, namely information, can be read by detecting the state of polarization of reflected light. Since a RAM disk can thus be rewritten, a user is capable of writing information such as text at will, unlike the case with a ROM disk. With a RAM disk, therefore, established information such as a system program and character fonts is recorded in a prescribed area of the disk, this area is made a write-inhibit area and other areas can be used as areas for recording user-created text, additional information and version upgrading information. However, a RAM disk requires that the established information be written thermomagnetically item by item. As a consequence, fabrication takes time and raises cost.
A partial ROM (a partial-ROM photomagnetic disk) has a ROM area whose structure is identical with that of a ROM disk, and a RAM area whose structure is identical with that of a RAM disk. As a result, fixed information such as a system program and character fonts can be recorded in the ROM area by stamping, thus eliminating the need to write the information item by item. In addition, the user is capable of writing text in the RAM area at will. In other words, a partial ROM is ideal for applications in which there is a need for an area (a ROM area) that stores fixed information as well as a rewritable area (a RAM area) on one and the same disk.
FIG. 14 is a diagram for describing the construction of a typical partial ROM. FIG. 14A is a schematic plan view, 14B a partially enlarged explanatory view of the partial ROM and 14C a partial sectional view of the same. In FIGS. 14A-14C, the partial ROM 11 has 10,000 tracks per side, in which the tracks are concentric circles or spiral in form. All of the tracks are divided into 25 sectors (25 blocks) ST. Each sector ST is composed of 512 bytes. The header of each sector ST is provided with an address field AF, with the rest of the sector being a data field DF. Address information is recorded in the address field AF and data is stored in the data field DF. The address information includes a sector mark, a track address, a sector address and a preamble for reproducing a synchronizing signal.
The outermost band and innermost band of the storage area of the partial ROM 11 is provided with a defect management area DMA. The defect management area DMA is provided with a disk definition se ctor DDS. Media type (i.e., whether the medium is a ROM or not), RAM area information and ROM area in formation are entered in the disk definition sector DDS.
The partial-ROM photomagnetic disk 11 has the above-described physical format. The outer side of the disk is a ROM area (ROM section) ha, and the inner side is a RAM area (RAM section) lb. As shown in FIG. 14C, the partial ROM 11 is composed of a transparent plastic layer PLS, in which pits PT are formed in part of the ROM area by stamping, a magnetic film MGF deposited on the plastic layer PLS, and a protective layer PRF formed on the magnetic film MGF. The ROM area 11a and the address fields AF are formed by stamping, fixed information such as a system program and character fonts is recorded in the ROM area 11a in the form of the pits PT, and address information is recorded in the address fields AF in the form of pits. A track guide groove TRG (see FIG. 14B) used in a tracking servo also is formed by stamping. The RAM area 11b and the defect management areas DMA are formed by coating the entire surface of the plastic layer PLS with the photomagnetic film MGF. In this case, the magnetic film is formed on the ROM area 11a as well but the inner circumferential portion other than the ROM area serves as a RAM area. The reading of information from the ROM area 11a is performed by irradiating this area with the laser beam LB via the objective lens OL and detecting the returning light. The writing of information in the RAM area 11b is performed by applying a magnetic field using a write coil (not shown) and irradiating the portion in which information is to be written with the laser beam LB via the objecting lens OL. The reading of information from the RAM area 11b is performed by utilizing the fact that the plane of polarization is rotated in the opposite direction, in conformity with the direction of magnetization, owing to the magnetic Kerr effect.
As mentioned above, the defect management areas DMA are provided with the disk definition sector DDS, in which media type (i.e., whether the medium is a ROM or not), RAM area information and ROM area information, etc., are entered. FIG. 15 is a diagram for describing the disk definition sector DDS. The sector is provided with a DDS identifier space 12a, a space (media type) 12b indicating whether the medium is a ROM disk or not, a RAM-area information space 12c, a ROM-area information space 12d, and a space 12e indicating the starting address of the defect management area. The following is entered in the RAM-area information space 12c: 1 group count 12c-1, namely the number of RAM groups obtained when the RAM area is divided into a plurality of groups; 2 data sector count (user block count) 12c-2, namely the number of data sectors in each RAM group; and 3 spare sector count (spare block count) 12c-3, namely the number of spare sectors used as substitutes in a case where a failure develops in a user block. A count 12d-1 of ROM groups and a count (user block count) 12d-2 of data sectors in each ROM group are entered in the ROM-area information space 12d.
A section is provided with a file management area for storing file management data and a file area for storing files. FIG. 16 is a diagram for describing the structure of a section. Numeral 13 denotes the section, 13a a file management area and 13b a file area. The following is stored in the file management area 13a: a disk descriptor 13a-1; redundant first and second space allocation tables (file allocation tables, abbreviated to "FAT") 13a-2, 13a-3; and a directory (information indicative of table of contents) 13a-4 designating the first cluster number of each file.
The disk descriptor 13a-1 describes the volume structure parameters of the disk, namely sector size (number of bytes per sector) SS, count SC of sectors (blocks) per cluster, count FN (=2) of FATs, count RDE of entries in a root directory, total count TS of sectors, count SF of sectors per FAT, and count SPT of sectors per track.
The FATs 13a-2, 13a-3 are each constituted by a format identifier (FI) 14a and a FAT entry portion 14b. When the content of the format identifier 14a is FD.sub.H (where H signifies hexadecimal notation), this means that the disk possesses a volume structure stipulated by ISO 7487. When the content of the format identifier 14a is F9.sub.H, this means that the volume structure parameters are specified by the disk descriptor 13a-1. The FAT entry portion 14b has FAT entries the number of which is equivalent to the number of clusters in the section. The FAT entries take on values of 0000, 0002.about.MAX, FFF7, FFFF, respectively, in which 0000 means that the cluster is not in use. Further, 0002.about.MAX mean that the cluster is in use, with the next storage location of a file being designated by the particular value. Further, FFF7 means that there is a defect in the sector constituting the cluster, and FFFF signifies end of file.
As shown in FIG. 17, each directory entry (32 bytes) in the directory 13a-4 has a space 15a for a file name, a space 15b for a file name extension, a space 15c for an attribute indication, a space 15d for a reserved field, a space 15e for file modification time, a space 15f for a file modification date, a space 15g for a starting cluster number of a file, and a space 15h for file length. FIG. 18 is a diagram for describing directory entries, which indicates the storage location of a file name "FILE", as well as FAT entries. It is assumed here that a file named "FILE" has been stored at cluster numbers 0004.sub.H.fwdarw.0005.sub.H.fwdarw.0006.sub.H.fwdarw.000A.sub.H. The starting cluster number "0004" of a file is stored at a directory entry in correlation with the file name "FILE". A cluster number "0004" indicating the next storage location of a file is stored at the FAT entry of cluster number 0005, a cluster number "0005" indicating the next storage location of a file is stored at the FAT entry of cluster number 0006, a cluster number "0006" indicating the final storage location of a file is stored at the FAT entry of cluster number 000A, and a cluster number "FFFF" indicating end of file is stored at the FAT entry of cluster number 000A.
As shown in FIG. 19A, the partial ROM has the ROM area 11a and the RAM area 11b. There are cases in which these areas are dealt with as independent sections (ROM section and RAM section). When the ROM area and RAM area are sections that are independent of each other, this is advantageous at the time of manufacture because file management information for the ROM section can be recorded in simple fashion by stamping. Moreover, files can be put in order independently section by section.
There are cases in file management in which certain users wish to handle a file (a ROM file) in the ROM section and a file (a RAM file) in the RAM section in a unified manner in the same level of the hierarchy. More specifically, there are cases in which a user may desire to treat all files (ROM files and RAM files) as files of a single united section that is a ROM-RAM mixture, as illustrated in FIG. 19B. However, in the conventional partial ROM in which the ROM area and RAM area are independent sections, the file management information of the ROM section is stored in the ROM area 11a and the file management information of the RAM section is stored in the RAM area 11b. This means that ROM files and RAM files cannot be handled in the same level of the hierarchy.
Further, there are instances in which file management information and ROM files are recorded in the ROM area by stamping and supplied by the manufacturer. In such instances there are occasions where it is desired that the entire partial ROM be made a partial ROM composed of a single united section that is a ROM-RAM mixture. However, a problem encountered is that the conversion to the single section cannot be made.